Book excerpt: This book has been written at a time when environmental issues and the move towards "clean technology" is driving synthetic chemists away from organic based solvent systems and towards water as the preferred medium of the future. The paints industry has already moved to aqueous based products. Metal aqua complexes are widely used in the areas of catalysis, dyes and pigments and in hydrometallurgy where a complete understanding of the metal ions in aqueous media is highly desirable.